The incorporation of electronic devices within pneumatic tyres is taking a greater importance in order to increase safety of vehicles. Tyre electronics may include sensors and other components suitable for obtaining information regarding various physical parameters of a tyre, such as for example temperature, pressure, number of tyre revolutions, etc. Such information may become useful in tyre monitoring and/or alarm systems. Furthermore, active control systems of the vehicle may be based on information sent from sensor devices included within the tyres. Typically, wireless transmission is employed in order to send tyre performance information outside the tyre, to a receiver disposed on the vehicle, so that such electronic devices disposed within the tyre typically include a transmitter associated to an antenna. A microprocessor is also typically employed, in order to collect and process the signals coming from the performance sensors, before transmission.
Such integrated tyre electronics have conventionally been powered by a variety of techniques and different power generation systems.
A typical solution for powering tyre electronics systems is the use of a non-rechargeable battery, which may cause inconveniences to a tyre user since proper electronics system operation is dependent on periodic battery replacement. As a matter of fact, batteries tend to deplete their energy storage quite rapidly when powering electronic applications characterized by complex levels of functionality. Furthermore, conventional batteries typically contain heavy metals that are not environmentally friendly and which present disposal concerns, especially when employed in numerous quantity. Moreover, performances of conventional batteries are often influenced by temperature: in particular, the functioning of such batteries is not reliable at low temperatures.
Another known method for powering tyre monitoring systems is a coupling of radio-frequency (RF) power between an antenna disposed on the vehicle in close proximity with an antenna included within the electronic device disposed in the tyre. This typically requires antennas disposed in vehicle portions frequently exposed to damage from road hazards, and thus may be an undesirable solution for powering tyre electronic applications.
The use of piezoelectric elements has also been proposed for powering tyre monitoring systems. Piezoelectricity is a property of certain materials, such as quartz, Rochelle salt, and certain solid-solution ceramic materials such as lead-zirconate-titanate (PZT), of generating electrical charge when mechanically stressed.
PCT patent application WO 2005/067073 discloses a tyre comprising a piezoelectric flexing element associated with an energy storage device (e.g. a capacitor). The piezoelectric flexing element is mounted in cantilever fashion in a housing so as to be positioned substantially along a plane orthogonal to a radial direction of said tyre and so that a first end of the piezoelectric element is restrained to the housing. A loading mass is coupled to the second end of the piezoelectric flexing element. A small gap is formed between the inner walls of the housing and the outer surface of the loading mass, in order to allow limited flexure of the piezoelectric element. The housing including the piezoelectric element is mounted in a tyre portion in correspondence of a tread area of the tyre, preferably on the inner surface of the tyre. The piezoelectric element flexes under the action of the radial acceleration when the tyre rotates. The loading mass and the gap are chosen to obtain: a) small entity oscillations of the flexure element substantially during a complete revolution of the tyre, when the tyre rotates at low speed; b) large entity oscillations of the flexure element substantially only during the passage of the tyre portion including the piezoelectric element in the contact patch.
PCT patent application WO 2006/003052 discloses a piezoelectric generator located on the inside of a tyre, where a piezoelectric flexible beam is mounted on a support. In order to guarantee the freedom of movement of the flexible beam, a housing is positioned over it, whilst a rigid sealing compound mechanically protects the overall arrangement on the inside of the tyre. As a result of the centripetal acceleration variations, the ends of the flexible beam are moved and deform the flexible beam. As a result, electrical energy is generated by the piezoelectric effect.
PCT patent application WO 2006/072539 discloses a configuration of a spring element as a rod spring, torsion spring or leaf spring, in which the free end of the spring element carries a seismic mass, to which an impulse is applied by the rolling of the tyre. The combination of the seismic mass with the spring element produces a spring-mass oscillator. Oscillation is generated by the motion whereby the tyre module, during the rotation of the tyre, describes a straight line as it passes through the contact area, and a circular path as it moves out of the contact area. In the circular path, centrifugal force acts on the seismic mass whereas, under ideal conditions, no force will act on the seismic mass in the contact area. Centrifugal force will displace the spring-mass oscillator, which will then return to its position of rest as it passes through the contact area.